(1) Field of the Invention
The present invention relates to an electron beam exposure apparatus, more particularly to an electron beam exposure apparatus having a control apparatus wherein transient response characteristics of an electromagnetic deflection means do not cause positioning errors of the electron beam.
(2) Description of the Prior Art
In the past, electron beam exposure has been effected by scanning a very small diameter electron beam along a certain pattern. Recently, a new electron beam exposure method has been developed in which two plates having rectangular apertures are arranged in the electron beam path and a deflection means, located between the plates, is used to control the passage of the electron beam therethrough, thereby obtaining an electron beam of any desired rectangular area. This new method has significantly reduced the time required for exposure and can be computer controlled.
In electron beam scanning, only the sections requiring exposure are scanned. This scanning method offers excellent scanning efficiency, but also has the disadvantage that the transient response characteristics of the deflection means, due to eddy current or the like, causes positioning errors of the electron beam.
Such transient response characteristic positioning errors occur when the magnetic material around the deflection coil for the electron beam has a memory function. More specifically, fluctuations of the deflecton speed of the electron beam occur, and the irradiation point of the electron beam moves to a point different than the desired one. For this reason, various attempts have been made to reduce positioning errors.
Up until now, it has only been possible to scan an area of several square millimeters at one time by an electron beam, even with an electron beam exposure apparatus using an electron beam having a rectangular section. Therefore, when a silicon wafer 1 has several hundred to several thousand chips 2, as shwn in FIG. 1, the silicon wafer 1 is mechanically moved in an LX direction and an LY direction by an XY stage for irradiation. Recently, it has become possible to scan a chip area of approximately 10 mm.times.10 mm (l.sub.x .times.l.sub.y) without moving the XY stage. Further, sometimes a beam 3 is moved to an adjacent chip, i.e., a subfield, a beam 3 is jumped one or two subfields which do not have to be irradiated, or a beam 3 is moved to a subfield considerably distant from a previous position. The scanning in each subfield is performed using an electrostatic deflection means of the electron beam exposure apparatus and the deflection of the electron beam from one subfield to another is performed using an electromagnetic deflection means.
During deflection between subfields by an electromagnetic deflection means, a heavy current is applied to electromagnetic deflection coils in order to move the electron beam a longer distance. This creates an eddy current in the metals around the deflection means and in wires of the deflection coils. As shown in FIG. 2, in the absence of an eddy current, the electron beam would move from a center A of a first subfield 2a to a center C of a third subfield 2c jumping over a second subfield 2b. In the presence of an eddy current, the electron beam is moved to a point C' by the change of deflection speed and an error e appears.
To reduce the error, litz wire, which comprises numerous fine wires twisted together, has been used as the electromagnetic deflection coils to decrease the eddy current effect. Also, as shown in FIG. 3, the inner wall 4 of an electron lens has been made of a magnetic material such as ferrite. These efforts have improved the electromagnetic and electrostatic deflection of the electron beam 7 to reach through an aperture 4a and through the centers of the electromagnetic deflection coils 5 and electrostatic deflector 6 to accurately enable predetermined positioning of the wafer 1 on an XY stage 8.
Such mechanical improvements, however, have not been able to completely eliminate error. Therefore, to prevent the eddy current in the deflection coils from having a detrimental effect, it is necessary to provide an idle time until the eddy current disappears. The thousands or tens of thousands of idle times increase the exposure time required for one wafer.